1. Field of the Invention
The present invention relates to a process for preparing N,N'-disubstituted urea which is useful for agricultural chemicals, herbicides, pesticides and carbamates.
More particularly, the present invention relates to an improved process for preparing disubstituted urea by reacting an amine with a mixture of carbon monoxide and oxygen in the presence of a catalyst composition consisting of at least one selenium compound as a main catalyst and an alkali metal carbonate as a co-catalyst.
2. Description of the Prior Art
The conventional process for preparing N,N'-disubstituted urea from amine and phosgene has disadvantages in that phosgene is a lethal and highly corrosive compound, and the method generates much HCl, a known pollutant.
R. A. Franz, J. Org. CHEM., vol 26, p3309 (1961) teaches a process for preparing urea by reacting an amine with CO and S in the presence of a tertiary amine as a catalyst, but by using sulfur, the process generates by-products, such as H.sub.2 S which is difficult to treat.
Japanese Patent Publication No. Sho 62-59,253 discloses a method for preparing urea from nitro compounds in the presence of catalysts such as rhodium and ruthenium at a relatively high turnover rate and selectivity. However, such catalysts of expensive precious metals may be disadvantageously decomposed at a high temperature and high reaction pressure.
Koyochi Kondo, Chemistry Letter, p373(1972) discloses a process of preparing urea by carbonylating an amine in the presence of a selenium catalyst. However, since the process uses an equivalent amount of selenium, a significant catalyst loss occurs. Furthermore, the reaction does not proceed when an aromatic amine is used as a starting material.
U.S. Pat. No. 4,052,454 discloses a process for preparing urea by reacting nitro compounds with water and CO in the presence of a selenium catalyst. However, as disclosed in its first embodiment, the process suffers from certain disadvantages, i.e., it is non-economical. Namely, when the reaction proceeds at about 1/8 molar fraction of catalyst to nitrobenzene as starting materials at 150.degree. C. and 53 atm, the conversion of nitrobenzene and yield of the urea are 66.3% and 33.8%, respectively (turnover number, defined as molar urea produced per unit time and unit molar catalyst, &lt;2).
European Patent No. 0 319,111 teaches a process for preparing urea from an amine and nitrobenzene wherein the activity of a palladium catalyst is maintained by adding redox agents, such as copper, manganese, vanadium, and chromium. However, as disclosed in its first embodiment, when the reaction proceeds at 140.degree. C. and 50 atm for 20 hours, the maximum yield of 1,1-dimethyl-3-(4-chlorobenzene) urea is very low, i.e., 73% (turnover number &lt;4).
As described above, the conventional processes for preparing urea are insufficient for industrial purposes. Furthermore, when an aromatic amine is singly employed, the yield of urea is extremely low.